Electrohydraulic device for operating the valves of a combustion engine

ABSTRACT

Electrohydraulic device for operating an intake or exhaust valve of a combustion engine; the electrohydraulic device comprises a linear hydraulic actuator designed to move the valve axially from a closed position to a maximum opening position, and an electronic control hydraulic distributor designed to regulate the flow of pressurised liquid from and towards the linear hydraulic actuator in order to control movement of the above-mentioned valve between the closed and maximum opening positions; the electronic control hydraulic distributor comprises a slide valve which, by selection, can be set to three positions: a first operating position in which it establishes direct communication between the linear hydraulic actuator and an outlet of the pressurised liquid, a second operating position in which it isolates the linear hydraulic actuator in such a way as to prevent the flow of pressurised liquid from or towards the actuator, and a third operating position which establishes direct communication between the linear hydraulic actuator and an inlet of the pressurised liquid.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims the priority of Italian Application No.BO2001A 000092 filed Feb. 20, 2001, the disclosure of which is beingincorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention concerns an electrohydraulic device foroperating the valves of a combustion engine.

[0003] As is known, combustion engines are currently being tested out inwhich the intake and exhaust valves that selectively establishcommunication between the engine combustion chamber and the engineintake and exhaust manifolds respectively are operated byelectrohydraulic devices driven by an electronic control unit.

[0004] This solution permits very accurate variation of the opening andclosing moments of the valves according to the angular speed of thecrankshaft and other engine operating parameters, considerablyincreasing engine efficiency.

[0005] In greater detail, combustion engines are currently being testedout provided with an electrohydraulic operating device for each engineintake and/or exhaust valve; said device comprises a linear hydraulicactuator designed to move the valve axially from the closed position tothe maximum opening position, overcoming the action of an elasticelement designed to maintain the valve in the closed position, and anelectronic control hydraulic distributor designed to regulate the flowof pressurised oil from and towards the hydraulic actuator, in such away as to control movement of the valve between the closed and maximumopening position.

[0006] To satisfy pressurised oil requirements, the combustion enginescurrently being tested are furthermore provided with a hydraulic circuitthat comprises an oil collection tank, inside which the oil to beconveyed to the actuators is stored at ambient pressure, and a pumpingunit designed to convey pressurised oil to the various hydraulicdistributors, taking it directly from the collection tank.

[0007] Each electronic control hydraulic distributor is connected to thehydraulic circuit in such a way as to establish direct communication,respectively, between the corresponding linear hydraulic actuator andthe pumping unit delivery inlet when it is necessary to move the valvefrom the closed position to the maximum opening position, and thecollection tank when it is necessary to move the valve from the maximumopening position to the closed position. In the first case, thepressurised oil is conveyed into the linear hydraulic actuator whereasin the second case the pressurised oil that fills the linear hydraulicactuator is conveyed directly into the collection tank.

[0008] In other words, therefore, all the pressurised oil conveyedinside the hydraulic actuator during movement of the valve from theclosed position to the maximum opening position is discharged directlyinto the collection tank during movement of the valve from the maximumopening position to the closed position, propelled by the elasticelement designed to keep the valve in the closed position.

[0009] The main disadvantage of the solution described above is theconsiderable amount of pressurised oil required which increasesproportionally to the engine rpm, and which calls for the use of pumpingunits that are so bulky as to be incompatible with use in automotiveapplications.

[0010] To solve the above problem, the applicant has developed andpatented a combustion engine in which the electrohydraulic operatingdevice is able to re-convey, during movement of the valve from themaximum opening position to the closed position, the majority of thepressurised oil present inside the hydraulic actuator into the highpressure part of the hydraulic circuit, exploiting the elastic energyaccumulated by the elastic element designed to keep the valve in theclosed position.

[0011] In other words, the linear hydraulic actuator and thecorresponding electronic control hydraulic distributor are constructedand driven in such a way as to re-pump, during movement of the valvefrom the maximum opening position to the closed position, the majorityof the pressurised oil present inside the hydraulic actuator into thehigh pressure part of the hydraulic circuit, exploiting the elasticenergy accumulated by the elastic element designed to keep the valve inthe closed position.

[0012] In this way, the overall pressurised oil requirements aredrastically reduced, making it possible to use small pumping unitscompatible with automotive use. The pressurised oil re-pumped by eachlinear hydraulic actuator into the high pressure part of the hydrauliccircuit can be immediately reused.

[0013] Unfortunately, the last solution described above requires the useof particularly complicated electrohydraulic devices which are notcurrently compatible with the automotive sector due to cost andreliability.

SUMMARY OF THE INVENTION

[0014] The aim of the present invention is to produce anelectrohydraulic device to operate the valves of a combustion enginewhich is more reliable and cheaper to produce than those currentlyknown, so that it can be effectively used in the automotive sector.

[0015] According to the present invention, an electrohydraulic device isproduced for the operation of at least one intake or exhaust valve of acombustion engine; the electrohydraulic device comprises a linearhydraulic actuator, which is designed to move said valve axially from aclosed position to a maximum opening position, and an electronic controlhydraulic distributor designed to regulate the flow of pressurisedliquid from and towards the linear hydraulic actuator, in such a way asto control movement of said valve between said closed and maximumopening positions; the electrohydraulic device is characterised in thatsaid electronic control hydraulic distributor comprises a slide valvewhich, by selection, can be set to a first operating position in whichit establishes direct communication between said linear hydraulicactuator and an outlet of the pressurised liquid, a second operatingposition in which it isolates said linear hydraulic actuator in such away as to prevent the flow of pressurised liquid from or towards theactuator, and a third operating position in which it establishes directcommunication between said linear hydraulic actuator and an inlet of thepressurised liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention will now be described with reference to theattached drawings which illustrate a non-restrictive implementationexample in which:

[0017]FIG. 1 illustrates schematically, with parts in section and partsremoved for clarity, a combustion engine equipped with electrohydraulicdevices for operation of the valves constructed according to the presentinvention;

[0018]FIG. 2 is an enlarged view, with parts in section and partsremoved for clarity, of one of the electrohydraulic devices for drivingthe valves of a combustion engine illustrated in FIG. 1; while

[0019]FIGS. 3, 4 and 5 illustrate in section the electrohydraulic deviceof FIG. 2 in three different operating positions.

DETAILED DESCRIPTION OF THE INVENTION

[0020] With reference to FIGS. 1 and 2, number 1 indicates overall anelectrohydraulic device designed to move by command at least one intakeor exhaust valve 2 of a combustion engine.

[0021] As is known, in fact, any combustion engine currently comprises:a engine block, one or more pistons fitted axially and sliding insiderespective cylindrical cavities made in the body of the engine block,and a head 3 positioned at the top of the engine block to close theabove-mentioned cylindrical cavities.

[0022] Together with the head 3, each piston defines, inside therespective cylindrical cavity, a variable volume combustion chamber 4which is connected to the engine intake manifold and exhaust manifold(both of known type and not illustrated) via at least one intake pipeand at least one exhaust pipe respectively, both made in the body of thehead 3; the combustion engine furthermore comprises a series of intakeand exhaust valves 2 designed to regulate the flow of air or burnt gasesflowing from and towards each combustion chamber 4 via the correspondingintake pipe and the corresponding exhaust pipe.

[0023] In greater detail, the intake and exhaust valves 2 are positionedin the head 3 corresponding to the inlet of each intake pipe and eachexhaust pipe, and move between a closed position, in which they preventpassage of the gases through the intake or exhaust pipe from and towardsthe combustion chamber 4, and a maximum opening position, in which theypermit passage of the gases through the intake or exhaust pipe from andtowards the combustion chamber with the maximum flow rate possible.

[0024] For each valve 2, the combustion engine also comprises arespective elastic element 5 designed to keep the valve 2 in the closedposition.

[0025] With reference to FIG. 1, in the example illustrated, each intakeor exhaust valve 2 is mushroom-shaped and fitted on the head 3 of theengine with its stem 2 a sliding axially through the body of the head 3and its head 2 b moving axially at the intake or exhaust pipe inlet, insuch a way as to move between a closed position, in which the head 2 bof the valve 2 prevents passage of the gases through the intake orexhaust pipe from and towards the combustion chamber 4, and a maximumopening position in which the head 2 b of the valves 2 protrudes insidethe combustion chamber 4, in such a way as to permit passage of thegases through the intake or exhaust pipe from and towards the combustionchamber 4 with the maximum flow rate possible.

[0026] As regards the elastic element 5, it consists of a compressionpre-loaded helical spring 5 fitted on the stem 2 a of the valve 2 sothat the first end stops against the head 3 of the engine and the secondend stops against a locating ring nut 2 integral with the stem 2 a ofthe valve 2 itself.

[0027] With reference to FIGS. 1 and 2, the electrohydraulic device 1for operation of the valves 2 is provided with an inlet, via which thepressurised oil is supplied to the electrohydraulic device 1, and anoutlet via which the pressurised oil flows out of the electrohydraulicdevice 1, and comprises a linear hydraulic actuator 10, designed to movethe valve 2 axially from the closed position to the maximum openingposition, overcoming the action of the elastic element 5, and anelectronic control hydraulic distributor 11 designed to regulate theflow of pressurised oil from and towards the hydraulic actuator 10, sothat it controls movement of the valve 2 between said closed and maximumopening positions.

[0028] The linear hydraulic actuator 10 consists, in the exampleillustrated, of a simple single-acting hydraulic piston while thehydraulic distributor 11 comprises: a slide valve 12, selectively ableto establish direct communication between the hydraulic actuator and thepressurised oil inlet or the pressurised oil outlet, or isolate thehydraulic actuator 10 from both inlet and outlet; an elastic element 13designed to keep the slide valve 12 in a first operating position, inwhich the valve itself establishes direct communication between thelinear hydraulic actuator 10 and the pressurised oil outlet; and anelectric control actuator 14 designed to move, by command, the slidevalve 12 from the first operating position, overcoming the action of theelastic element 13.

[0029] In greater detail, the electric control actuator 14 is designedto move, by command, the slide valve 12 from a first operating positionto a second operating position, in which the slide valve 12 isolates thelinear hydraulic actuator 10 from the pressurised oil inlet and outlet,passing through a third operating position in which the valveestablishes direct communication between the linear hydraulic actuator10 and the pressurised oil inlet.

[0030] With reference to FIGS. 1 and 2, in the example illustrated, thelinear hydraulic actuator 10 and the hydraulic distributor 11 areintegrated in one single structure, and the electrohydraulic device 1therefore comprises:

[0031] an outer casing 15 designed to be fixed to the head 3 immediatelyabove the intake or exhaust valve 2 operated by the electrohydraulicdevice 1;

[0032] a piston 16, fitted axially to slide inside a cylindrical cavity17 that extends inside the outer casing 15 so that it is coaxial withthe axis A of the stem of the valve 2;

[0033] a slider 18 fitted axially to slide inside a cylindrical cavity19 that extends inside the outer casing 15 beside the cylindrical cavity17, so that it is coaxial with an axis B preferably but not necessarilyparallel to the axis A;

[0034] a helical spring 20 coaxial with the axis B inside thecylindrical cavity 19 with the two ends stopping, respectively, againstone of the two end surfaces of the cavity and against the axial end ofthe slider 18, in order to keep the latter positioned firmly against theother end surface of the cylindrical cavity 19, hereinafter referred toby number 19 a; and finally

[0035] a second piston 21, fitted axially to slide inside a cylindricalcavity 22 which extends inside the outer casing 15 coaxially to axis B,from the end surface of the cylindrical cavity 19 against which theslider 18 is pushed by the helical spring 20, or from the end surface 19a.

[0036] With reference to FIG. 2, the cylindrical cavity 17 communicatesdirectly with the outside so that it faces the upper end of the stem 2 aof the valve 2, and the piston 16 is fitted in the cylindrical cavity 17so that it protrudes partially outside the cavity, or the outer casing15, thus positioning itself and remaining always with one end againstthe upper end of the stem 2 a of the valve 2.

[0037] The piston 16, furthermore, is fitted to move inside thefluid-tight cylindrical cavity 17, creating inside the latter a variablevolume chamber 17 a selectively designed to be filled with pressurisedoil. This pressurised oil is able to exert on the piston 16 a sufficientforce to overcome the action of the elastic element 5, and to axiallymove the piston 16 from a retracted position, in which it protrudesoutside the cylindrical cavity 17 by a set length H′, to an extendedposition in which it protrudes outside the cylindrical cavity 17 by aset length H″, greater than H′.

[0038] It should be noted that the piston 16, or the linear hydraulicactuator 10, since it is always positioned against the upper end of thestem 2 a of the valve 2, when it is in the retracted position sets thevalve 2 to the closing position whereas when it is in the extendedposition, it sets the valve 2 to the maximum opening position. Thedifference between the lengths H′ and H″ corresponds to the stroke orlift of the valve 2.

[0039] As regards the hydraulic distributor 11 and in particular theslide valve 12, the cylindrical cavity 19 is provided with a series ofexhaust ports which communicate, via a series of connection pipes madein the body of the outer casing 15, with the pressurised oil inlet 15 aand with the pressurised oil outlet 15 b, both made in the body of theouter casing 15, and with the variable volume chamber 17 a inside thecylindrical cavity 17 respectively. As regards the slider 18, it isfitted axially to slide inside the cylindrical cavity 19 in such a wayas to obstruct, according to its position inside the cavity, one or moreof the above exhaust ports, thus regulating the flow of pressurised oilfrom and towards the variable volume chamber 17 a of the linearhydraulic actuator 10.

[0040] With reference to FIG. 2, in particular, the cylindrical cavity19 is laterally defined by a cylindrical tubular liner 23 provided withthree annular exhaust ports axially distributed along the cylindricalside wall of the liner itself.

[0041] The first exhaust port, hereinafter referred to by number 23 a,is positioned at a distance da determined by the end surface of thecylindrical cavity 19 against which the slider 18 stops, or by the endsurface 19 a, and is connected to the variable volume chamber 17 ainside the cylindrical cavity 17 via a first connection pipe. The secondexhaust port, hereinafter indicated by number 23 b, is positioned at adistance db determined by the end surface 19 a, and is connected to thepressurised oil inlet 15 a by means of a second connection pipe.Finally, the third exhaust port, hereinafter referred to by number 23 c,is positioned at a distance dc determined by the end surface 19 a, andis connected again to the variable volume chamber via a third connectionpipe.

[0042] It should also be noted that the three distances da, db and dcare assessed parallel to the axis B and are progressively increasing.

[0043] A fourth exhaust port, hereinafter indicated by number 23 d, ismade directly on the end of the cylindrical cavity 19 where one end ofthe helical spring 20 rests. Said fourth exhaust port communicatesdirectly with the pressurised oil outlet 15 b via a fourth connectionpipe.

[0044] With reference to FIG. 2, the slider 18 consists of a shapedpiston which is fitted axially to move inside the cylindrical tubularliner 23 between a first operating position (see FIG. 2), in which itstops against the end surface 19 a of the cylindrical cavity 19, and asecond operating position (see FIG. 5), in which it is positioned at amaximum pre-set distance from the end surface 19 a.

[0045] The slider 18, in particular, is fitted to move inside thefluid-tight cylindrical tubular liner 23, and is shaped in order toestablish direct communication between the exhaust ports 23 c and 23 dand prevent the exhaust ports 23 a and 23 b being in directcommunication with each other or with the exhaust port 23 d when it isin the first operating position. The slider 18, furthermore, is shapedin order to prevent the exhaust ports 23 a, 23 b and 23 c communicatingwith one another or with the exhaust port 23 d when it is in the secondoperating position, and in such a way as to temporarily establishcommunication between the exhaust ports 23 a and 23 b during movementfrom the first to the second operating position.

[0046] In the example illustrated, in particular, the shaped piston 18has an axial length L which approximates by defect the distance dcseparating the third exhaust port 23 c from the end surface 19 a of thecylindrical cavity 19, and is provided with an annular slot 18 a nearthe axial end facing the end surface 19 a of the cylindrical cavity 19.

[0047] This annular slot 18 a has a width G, measured parallel to theaxis B, that approximates by excess the distance between the exhaustports 23 a and 23 b (or approximates by excess the difference betweenthe distances db and da), in order to temporarily establish directcommunication between the exhaust port 23 a and the exhaust port 23 bduring axial movement of the shaped piston 18 inside the cylindricalcavity 19.

[0048] The annular slot 18 a, furthermore, is positioned on the shapedpiston body 18 in such a way as to keep the exhaust ports 23 a and 23 bisolated from each other when the shaped piston 18 is in the firstoperating position. In other words, the annular slot 18 a is positionedon the shaped piston body 18 in such a way as to face the exhaust port23 a, but not the exhaust port 23 b, when the shaped piston 18 stopsagainst the end surface 19 a of the cylindrical cavity 19.

[0049] With reference to FIG. 2, it should furthermore be underlinedthat the annular slot 18 a is positioned on the shaped piston body 18 sothat, at the end of the piston stroke, it overshoots the exhaust port 23a, but without simultaneously facing the exhaust ports 23 b and 23 c,thus avoiding establishing direct communication between the twoabove-mentioned exhaust ports.

[0050] In the light of the above, when the slider 18 is in the firstoperating position, the variable volume chamber 17 a of the linearhydraulic actuator 10 is in direct communication with the pressurisedoil outlet 15 b and the slide valve 12 is therefore in the firstoperating position.

[0051] When the slider 18 is in the second operating position, thevariable volume chamber 17 a of the linear hydraulic actuator 10 isisolated from the pressurised oil inlet 15 a and outlet 15 b, and theslide valve 12 is therefore in the second operating position.

[0052] During movement of the slider 18 from the first to the secondoperating position, the variable volume chamber 17 a of the linearhydraulic actuator 10 temporarily communicates with the pressurised oilinlet 15 a and the slide valve 12 is therefore in the third operatingposition.

[0053] Lastly, as regards the electric control actuator 14, withreference to FIG. 2, the cylindrical cavity 22 faces the axial end ofthe slider 18 facing the end surface 19 a, and the piston 21 is fittedin the cylindrical cavity 22 in such a way that it partially protrudesoutside the cavity so that it is positioned and remains with one endagainst the axial end of the slider 18.

[0054] The piston 21, furthermore, is fitted to move inside thefluid-tight cylindrical cavity 22 in order to create inside the latter avariable volume chamber 22 a selectively designed to be filled withpressurised oil. This pressurised oil is able to exert on the piston 21a force sufficient to overcome the action of the helical spring 20, orthe elastic element 13, and to axially move the piston 21 from aretracted position, in which it protrudes outside the cylindrical cavity22 by a set length K′, to an extended position in which it protrudesoutside the cylindrical cavity 22 by a set length K″, greater than K′.

[0055] Also in this case it should be pointed out that the piston 21, asit is always against the axial end of the slider 18, sets the slider 18to the first operating position when it is in the retracted position,whereas when it is in the extended position it sets the slider 18 to thesecond operating position. The difference between the lengths K′ and K″corresponds to the stroke that the slider 18 can travel inside thecylindrical cavity 19.

[0056] As regards inflow and outflow of the pressurised oil to/from thevariable volume chamber 22 a, the electric control actuator 14 isprovided with two solenoid valves with controlled opening and closing,fitted inside the outer casing 5, to regulate the pressurised oil inflowand outflow to/from the variable volume chamber 22 a.

[0057] In the example illustrated, in particular, the electric controlactuator 14 comprises two fuel injectors of known type, fitted in theouter casing 5 in such a way as to reach the variable volume chamber 22a. The first fuel injector, hereinafter indicated by number 25, has itsspray nozzle facing towards the variable volume chamber 22 a, and isdesigned to regulate the inflow of pressurised oil to the variablevolume chamber 22 a, while the second fuel injector (not visible as itis covered by the first one) faces in the opposite direction, or so thatthe spray nozzle faces away from the variable volume chamber 22, and isdesigned to regulate the outflow of pressurised oil from the variablevolume chamber 22 a.

[0058] It should be noted that the pressurised oil sent to the variablevolume chamber 22 a of the electric control actuator 14 can have apressure different from the pressurised oil that is sent to theelectrohydraulic device 1 through the inlet 15 a. In this way, it ispossible to regulate the lift of the valve 2 directly via the pressurevalue of the oil going into the electrohydraulic device 1 through theinlet 15 a: as the pressure increases, the lift of the valve 2 of theengine increases.

[0059] Operation of the electrohydraulic device 1 by activation of theintake or exhaust valves 2 of a combustion engine will now be describedwith reference to FIGS. 2, 3, 4 and 5, assuming that the valve 2 is inthe closed position, that the piston 16 is in the retracted position andthat the piston 21 and the slider 18 are in the retracted position andthe first operating position respectively.

[0060] When the command is given for opening of the fuel injector 25,the pressurised oil enters the variable volume chamber 22 a of theelectric control actuator 14 and gradually pushes the piston 21 out ofthe cylindrical cavity 22, overcoming the elastic force exerted by thehelical spring 20, so that it moves the slider 18 from the firstoperating position.

[0061] In the initial part of the stroke of the slider 18, the exhaustport 23 c is progressively closed by the body of the slider 18, whileexhaust ports 23 a and 23 b are kept isolated from each other. In otherwords, in the initial part of the stroke of the slider 18, the variablevolume chamber 17 a of the linear hydraulic actuator 10 is kept indirect communication with the pressurised oil outlet 15 b, and thepiston 16 therefore remains in the retracted position, leaving the valve2 in the closed position.

[0062] With reference to FIG. 3, at the end of this first part of thestroke of piston 21, the slider 18 has completely closed the exhaustport 23 c and is about to establish communication between the exhaustports 23 a and 23 b. At this moment, the variable volume chamber 17 a ofthe linear hydraulic actuator 10 is isolated from the pressurised oilinlet 15 a and outlet 15 b.

[0063] With reference to FIG. 4, in the middle part of the stroke ofpiston 21, the slider 18 establishes direct communication between theexhaust port 23 a and the exhaust port 23 b via the annular slot 18 aand the pressurised oil can therefore reach the variable volume chamber17 a of the linear hydraulic actuator 10 and gradually push the piston16 out of the cylindrical cavity 17, in order to gradually move thevalve 2 from the closed position to the maximum opening position,overcoming the elastic force of the elastic element 5.

[0064] With reference to FIG. 5, in the final part of the stroke ofpiston 21 that sets the piston 21 to the extended position and theslider 18 to the second operating position, the body of the slider 18gradually closes the exhaust port 23 a, until the variable volumechamber 17 a of the linear hydraulic actuator 10 is completely isolatedfrom the pressurised oil inlet 15 a.

[0065] In this condition, the exhaust ports 23 a, 23 b and 23 c are allclosed by the body of the slider 18 and the pressurised oil can nolonger enter or leave the variable volume chamber 17 a: consequently thepiston 26 remains blocked in the extended position and the valve 2 inthe maximum opening position.

[0066] At this point, the fuel injector 25 is cut off, or closed, inorder to block the piston 21 in the extended position.

[0067] The electrohydraulic device 1 can keep the valve 2 in the maximumopening position indefinitely until the other fuel injector is supplied,permitting outflow of the pressurised oil from the variable volumechamber 22 a of the electric control actuator 14 and consequent gradualreturn of the slider 18 to the first operating position, pushed by thehelical spring 20.

[0068] In the movement from the second to the first operating position,the slider 18 obviously permits re-pumping of the majority of thepressurised oil contained in the variable volume chamber 17 a of thelinear hydraulic actuator 10 towards the pressurised oil inlet 15 a.

[0069] The electrohydraulic device 1 for activation of the intake orexhaust valves 2 has the considerable advantage of featuring aparticularly simple structure that guarantees a high level ofreliability in the long term, therefore permitting use in the automotivesector. Furthermore, the electrohydraulic device 1 is relativelyinexpensive to produce.

[0070] Lastly it is clear that modifications and variations can be madeto the electrohydraulic device 1 described here while remaining withinthe scope of the present invention.

[0071] The invention has been described in detail with respect topreferred embodiments, and it will now be apparent from the foregoing tothose skilled in the art, that changes and modifications may be madewithout departing from the invention in its broader aspects, and theinvention, therefore, as defined in the appended claims, is intended tocover all such changes and modifications that fall within the truespirit of the invention.

What is claimed is:
 1. Electrohydraulic device (1) for operating anintake or exhaust valve (2) of a combustion engine; the electrohydraulicdevice (1) comprises a linear hydraulic actuator (10) designed to movethe valve (2) axially from a closed position to a maximum openingposition, and an electronic control hydraulic distributor (11) designedto regulate the flow of pressurised liquid from and towards the linearhydraulic actuator (10) in order to control movement of theabove-mentioned valve (2) between said closed and maximum openingpositions; the electrohydraulic device (1) is characterised in that saidelectronic control hydraulic distributor (11) comprises a slide valve(12) which, by selection, can be set to three positions: a firstoperating position in which it establishes direct communication betweensaid linear hydraulic actuator (10) and an outlet (15 b) of thepressurised liquid, a second operating position in which it isolatessaid linear hydraulic actuator (10) in such a way as to prevent the flowof pressurised liquid from or towards the actuator, and a thirdoperating position which establishes direct communication between saidlinear hydraulic actuator (10) and an inlet (15 a) of the pressurisedliquid.
 2. Electrohydraulic device according to claim 1, characterisedin that said electronic control hydraulic distributor (11) compriseselectronic control movement devices (13, 14) designed, by selection, tomove said slide valve (12) between said first, said second and saidthird operating position.
 3. Electrohydraulic device according to claim2, characterised in that said electronic control movement devices (13,14) comprise an elastic element (13) designed to keep said slide valve(12) in the first operating position, and an electric control actuator(14) designed to move, by command, said slide valve (12) from said firstoperating position to said second operating position, overcoming theaction of the elastic element (13); in the movement from said first tosaid third operating position, said electric control actuator (14) isdesigned to position said slide valve (12) in said third operatingposition.
 4. Electrohydraulic device according to claim 2, characterisedin that said slide valve (12) comprises an outer casing (15) and aslider (18) fitted axially to slide inside a first cylindrical cavity(19) which extends inside said outer casing (15); said first cylindricalcavity (19) is provided with a series of exhaust ports (23 a, 23 b, 23c, 23 d) which communicate directly with said pressurised liquid inlet(15 a), said pressurised liquid outlet (15 b) and said linear hydraulicactuator (10); the slider (18) is fitted axially to slide inside saidfirst cylindrical cavity (19) in such a way as to obstruct, according toits position inside the cavity, one or more of the abovementionedexhaust ports (23 a, 23 b, 23 c, 23 d) in order to regulate the flow ofpressurised liquid from and towards said linear hydraulic actuator (10).5. Electrohydraulic device according to claim 4, characterised in thatsaid elastic element (13) comprises a helical spring (20) positionedinside said cylindrical cavity (19) with the two ends resting against afirst end surface of the first cylindrical cavity (19) and the axial endof the slider (18) respectively, in such a way as to keep the latterfirmly resting against a second end surface (19 a) of said firstcylindrical cavity (19).
 6. Electrohydraulic device according to claim4, characterised in that said electric control actuator (14) comprises asecond piston (21), fitted axially to slide inside a second cylindricalcavity (22) which extends inside said outer casing (15) coaxially withsaid first cylindrical cavity (19) from said second end surface (19 a)of the first cylindrical cavity (19), against which said slider (18) ispushed; said second piston (21) is fitted to move inside said secondfluid-tight cylindrical cavity (22) in such a way as to define insidethe latter a variable volume chamber (22 a) selectively designed to befilled with pressurised liquid.
 7. Electrohydraulic device according toclaim 1, characterised in that said linear hydraulic actuator (10)comprises an outer casing (15) and a third piston (16) fitted axially toslide inside a third cylindrical cavity (17) which extends in the outercasing (15) coaxially with the stem (2 a) of the valve (2) and faces theupper end of said stem (2 a); said third piston (16) is fitted in thethird cylindrical cavity (17) in such a way as to protrude partiallyoutside the cavity, thus positioning itself and remaining always againstthe upper end of the stem (2 a) of the valve (2), and is fitted to moveinside said third fluid-tight cylindrical cavity (17) in such a way asto define inside the latter a variable volume chamber (17 a) selectivelydesigned to be filled with pressurised liquid.